Scientists and engineers have long sought to improve filtration performance of air filters. Some of the most effective air filters use electret articles. Electret articles exhibit a persistent or quasi-permanent electrical charge. See G. M. Sessler, Electrets, Springer Verlag, N.Y., 1987. Researchers have expended considerable efforts to improve the properties of electret articles for use in filters. Despite extensive research directed toward producing improved electret articles, the effects of processing variables are not well understood, and, in general, the effects of varying processing conditions are difficult if not impossible to predict.
Electret articles have special property requirements such as charge stability, loading performance, resistance to moisture and oil exposure, et cetera, that can be significantly affected by processing steps that would be generally innocuous or beneficial for nonwoven fabrics and fabric-like materials. Thus, in the absence of extensive empirical data, it is often very difficult to understand the effects that a particular processing step (for example quenching) might or might not have on the resulting product's electret properties.
One method that has been reported to improve electret filter performance is blending a performance-enhancing additive into a polymer that is used to form electret fibers. For example, Jones et al. in U.S. Pat. Nos. 5,411,576 and 5,472,481 disclose electret filters that are made by extruding a blend of polymer and a melt-processable fluorochemical to form a microfibrous web that is subsequently annealed and corona treated. Lifshutz et al. in WO 96/26783 (corresponding to U.S. Pat. No. 5,645,627) report electret filters that are made by extruding a blend of polymer and a fatty acid amide or a fluorochemical oxazolidinone fluorochemical to form a microfibrous web that is subsequently annealed and corona treated.
Other techniques have been reported which improve an electret article's charge properties. For example, Klaase et al. in U.S. Pat. No. 4,588,537 report using corona treatment to inject charge into an electret filter. Angadjivand et al. in U.S. Pat. No. 5,496,507 found that impinging water droplets onto a nonwoven microfiber web imparted a charge to the web, and Rousseau et al. in WO 97/07272 disclose electret filters that are made by extruding blends of a polymer and a fluorochemical or organic triazine compound to form a microfibrous web that is subsequently impinged with water droplets to impart charge and thereby improve the hydrocharged web's filtration performance.
Matsuura et al. in U.S. Pat. No. 5,256,176 disclose a process of making stable electrets by exposing an electret to alternating cycles of applying electric charges and subsequently heating the article. Matsuura et al. do not disclose electrets having additives that enhance oily mist loading performance.